For instance, the wheels of the car are rotatably supported with the suspension system via the double row angular contact ball bearing unit. Also, in order to secure the running stability of the car, the vehicle running stabilizing system such as the anti-lock brake system (ABS) or the traction control system (TCS), the vehicle stability control system (VSC), or the like is employed. In order to control such vehicle running stabilizing system, various signals such as rotational speeds of the wheels, accelerations applied to the car body from respective directions, and so on are required. Then, In order to execute higher control, in some cases it is preferable to know a magnitude of the load (one or both of the radial load and the axial load) applied to the rolling bearing unit via the wheel.
In light of such circumstances, in JP-A-2001-21577, the rolling bearing unit equipped with the load measuring unit capable of measuring the radial load is set forth. The rolling bearing unit equipped with the load measuring unit in this first example in the related art measures the radial load and is constructed as shown in FIG. 17. A hub 2 that is a rotating ring, to which the wheel is coupled and fixed, and corresponds to an inner ring equivalent member is supported on the inner diameter side of an outer ring 1 that is a stationary ring, which is supported with the suspension system, and corresponds to an outer ring equivalent member. This hub 2 has a hub main body 4 and an inner ring 6. Such hub main body 4 has a rotary-side flange 3, to which the wheel is fixed, on its outer end portion in the axial direction (where “the outside in the axial direction” means the outside in the width direction in the fitted state to the vehicle. This is true throughout the present specification and the claims.). Such inner ring 6 is fitted onto the inner end portion of this hub main body 4 (where “the inside in the axial direction” means the center side in the width direction in the fitted state to the vehicle. This is true throughout the present specification and the claims.), and then fixed with nuts 5. Then, a plurality of rolling elements 9a, 9b are arranged respectively between double row outer ring raceways 7, 7 formed on the inner peripheral surface of the outer ring 1 as the stationary-side raceway respectively and double row inner ring raceways 8, 8 formed on the outer peripheral surface of the hub 2 as the rotating-side raceway respectively, so that the hub 2 can be rotated on the inner diameter side of the outer ring 1.
A fitting hole 10 passing through the outer ring 1 in the diameter direction is formed in the middle portion of the outer ring 1 between the double row outer ring raceways 7, 7 along the axial direction in the direction almost perpendicular to the top end portion of this outer ring 1. Then, a circular lever-like (rod-like) displacement sensor 11 serving as a load measuring sensor is fitted into the fitting hole 10. This displacement sensor 11 is of non-contact type, and a sensing surface provided to a top end surface (lower end surface) is faced closely to an outer peripheral surface of a sensor ring 12 that is fitted and fixed onto the middle portion of the hub 2 in the axial direction. The displacement sensor 11 outputs a signal in response to an amount of change when a distance between the sensing surface and the outer peripheral surface of the sensor ring 12 is changed.
In the case of the rolling bearing unit equipped with the load measuring unit constructed as above in the related art, the load applied to the rolling bearing unit can be derived based on the sensed signal of the displacement sensor 11. More particularly, the outer ring 1 supported with the suspension system of the vehicle is pushed downward by the weight of the vehicle whereas the hub 2 onto which the wheel is supported and fixed tends to still remain in that position as it is. For this reason, a displacement between a center of these outer rings 1 and a center of the hub 2 is increased larger based on elastic deformations of the outer ring 1 and the hub 2 and the rolling elements 9a, 9b whenever a weight is increased larger. Then, a distance between the sensing surface of the displacement sensor 11, which is provided to the top end portion of the outer ring 1, and the outer peripheral surface of the sensor ring 12 is shortened smaller as the weight is increased larger. Therefore, if a sensing signal of the displacement sensor 11 is sent to a controller, the radial load applied to the rolling bearing unit into which the displacement sensor 11 is incorporated can be calculated from a relational expression, a map, or the like derived previously by the experiment, or the like. According to the loads applied to respective rolling bearing units derived in this manner, not only can the ABS be properly controlled but also the failure of the carrying state can be informed of the driver.
In this case, the related-art structure shown in FIG. 17 makes it possible to sense the rotational speed of the hub 2 in addition to the load applied to the rolling bearing unit. For this purpose, a sensor rotor 13 fitted and fixed onto the inner end portion of the inner ring 6, and also a rotational speed sensor 15 is supported by a cover 14 attached to the inner end opening portion of the outer ring 1. Then, a sensing portion of the rotational speed sensor 15 is opposed to a sensed portion of the sensor rotor 13 via a sensed clearance.
In operation of this rolling bearing unit equipped with the rotational speed sensor as described above, an output of this rotational speed sensor 15 is changed when the sensor rotor 13 together with the hub 2 to which the wheel is fixed is rotated and then the sensed portion of the sensor rotor 13 passes through the close vicinity of the sensing portion of the rotational speed sensor 15. A frequency at which the output of the rotational speed sensor 15 is changed in this manner is in proportion to the number of revolution of the wheel. Therefore, the ABS or the TCS can be controlled appropriately by feeding an output signal of the rotational speed sensor 15 to the not-shown controller.
The rolling bearing unit equipped with the load measuring unit in the above first example of the related-art configurations is used to measure the radial load applied to the rolling bearing unit. Also, the structure to measure the axial load applied to the rolling bearing unit is set forth in JP-A-3-209016, and the like and known in the related art. FIG. 18 shows the rolling bearing unit equipped with the load measuring unit set forth in JP-A-3-209016 as mentioned the above and used to measure the axial load. In the case of the second example of the related-art configurations, a rotary-side flange 3a used to support the wheel is provided to an outer peripheral surface of an outer end portion of a hub 2a as the rotating ring and the inner ring equivalent member. Also, a stationary-side flange 17 used to support/fix an outer ring 1a to a knuckle 16 constituting the suspension system is provided to an outer peripheral surface of the outer ring 1a as the stationary ring and the outer ring equivalent member. Then, a plurality of rolling elements 9a, 9 are provided rotatably between the double row outer ring raceways 7, 7 formed on the inner peripheral surfaces of the outer ring 1a and the double row inner ring raceways 8, 8 formed on the outer peripheral surfaces of the hub 2a respectively, whereby the hub 2a can be supported rotatably on the inner diameter side of the outer ring 1a. 
In addition, a load sensor 20 is affixed to plural locations on the inner-side surface of the stationary-side flange 17 to surround a screwed hole 19, into which a bolt 18 is screwed to couple the stationary-side flange 17 to the knuckle 16, respectively. These load sensors 20 are put between the outer-side surface of the knuckle 16 and the inner-side surface of the stationary-side flange 17 in a state that the outer ring 1a is supported/fixed to the knuckle 16.
In the case of the load measuring unit for the rolling bearing unit in the above second example of the related-art configurations, the outer-side surface of the knuckle 16 and the inner-side surface of the stationary-side flange 17 push strongly against respective load sensors 20 mutually from both surfaces in the axial direction whenever the axial load is applied between the not-shown wheel and the knuckle 16. Therefore, the axial load applied between the wheel and the knuckle 16 can be sensed by summing up measured values derived from these load sensors 20. Also, in JP-B-62-3365, although not shown, the method of deriving the revolution speed of the rolling elements based on a vibration frequency of the outer ring equivalent member, a rigidity in a part of which is lowered, and also measuring the axial load applied to the rolling bearing is set forth.
In the case of the first example of the related-art structures shown in above FIG. 17, the load applied to the rolling bearing unit is measured by sensing displacements of the outer ring 1 and the hub 2 in the radial direction by means of the displacement sensor 11. In this case, because an amount of displacement in the radial direction is very minute, a high-precision sensor must be employed as the displacement sensor 11 to measure this load with good precision. Since a high-precision non-contact type sensor is expensive, it is inevitable that a cost of the rolling bearing unit equipped with the load measuring unit is increased as a whole.
Also, in the case of the second example of the related-art structures shown in above FIG. 18, the load sensors 20 must be provided as many as the bolts 18 that are used to support/fix the outer ring 1a to the knuckle 16. As a result, because of this situation together with the above situation that the load sensors 20 themselves are expensive, it is inevitable that a cost of the overall rolling bearing unit equipped with the load measuring unit is considerably increased. Also, because the rigidity in a part of the outer ring equivalent member must be lowered to apply the method set forth in JP-B-62-3365 as mentioned the above, it is possible that security of the durability of the outer ring equivalent member becomes difficult.